All cellular telephones contain an amplifier for amplifying transmit signal. Because cellular telephones are small in size and preferably contain a single rechargeable battery, the amplifier is preferred to operate with a single polarity power supply. Moreover, it is preferred that the amplifier consumes low power, particularly when the cellular telephones are in a "standby" mode. A cellular telephone is in a "standby mode" when it is able to receive a ring signal indicating an incoming call but cannot send a transmit signal unless it is activated. It is further preferred that the amplifier has high power capability, is durable and can be made at low cost.
Power amplifiers used in cellular phones typically include at least one transistor for signal amplification and a transistor biasing circuit. A known amplifier includes a discrete silicon power transistor such as a silicon bipolar transistor or an enhancement-mode silicon MOSFET, and a transistor biasing circuit which is not monolithically integrated with the power transistor. This amplifier has certain advantages: silicon technology is well understood and well developed; silicon power transistors are readily available; a discrete silicon transistor can be individually optimized for power or efficiency. It however suffers the following drawbacks.
First, as compared with GaAs transistors, silicon transistors are generally slower in speed and have lower power capability. Second, it is difficult to monolithically integrate a silicon power transistor with its biasing circuit, because a silicon integrated circuit ("IC") has large parasitics especially at high frequencies. The large parasitics are caused by the pn junctions used in a silicon ICs for device isolation. In comparison, device isolation in a GaAs IC is typically accomplished by simply forming each device on a mesa on a semi-insulating substrate, which results in lower parasitics.
Further, because of its lower electron mobility and velocity, silicon transistors need to be optimized for power. The optimized transistors generally have structures not compatible for monolithic integration. Consequently, a silicon power transistor used in a power amplifier is normally a discrete device, which results in increased assembly cost and reduced reliability.
GaAs based transistors are more preferable than silicon transistors for use in amplifiers because they provides higher power and are capable of operating at higher frequencies. Moreover, they can be monolithically integrated with a GaAs biasing circuit. Among GaAs field effect transistors ("FETs"), depletion-mode GaAs FETs are most suitable for cellular communications because they provide higher power and operate at higher frequencies than enhancement-mode GaAs FETs.
To bias a depletion-mode GaAs FET at a desired operating point, a negative bias must be applied between the gate and source of the FET. The amplitude of this bias depends on the particular application the amplifier is intended. For a class A amplifier, the FET is biased at one half of its zero-bias saturation current, I.sub.dss ; the FET conducts current for an entire input cycle and consumes power continuously. For a class AB amplifier, however, the depletion-mode FET is biased at one quarter of I.sub.dss so that the FET is "slightly on" when there is no input signal. For a Class B amplifier, which typically include two depletion-mode FETs arranged in a push-pull fashion such that only one of the FETs is turned on during any half cycle of an input sinusoidal RF signal, the two FETs are biased at pinch-off.
In addition to the negative bias, a depletion FET also requires a positive bias between its drain and source. Thus, a depletion-mode FET requires two biases to operate, a positive bias between the drain and source, and a negative bias between the gate and source. For a typical depletion-mode GaAs MESFET, the positive bias required is about 3 to 12 volts, and the negative bias is about -0.5 to -4 volts. To provide the positive and negative biases, one may use two power supplies, one for the positive bias and the other for the negative bias. It is preferred, however, to use only a single polarity power supply.
There are several known power amplifiers which operate with a single polarity power supply. FIG. 1 depicts a self-biased GaAs amplifier containing a depletion mode GaAs FET 10. The drain of the FET is connected to a positive dc voltage supply, +V.sub.DD, through an inductor 15; the source is connected to ground through a resistor 20; the gate is dc biased to ground through an inductor 25. A capacitor 30 is connected in parallel to resistor 20 for providing an ac path from the source to ground. An input ac signal is provided at the gate of the FET.
In this amplifier, resistor 20 operates to establish a negative bias between the gate and the source. More specifically, because the gate is grounded through inductor 25 and the source is at a positive potential which equals to the voltage on resistor 20, the gate is biased at a potential that is negative with respect to the source. The amplitude of the negative bias approximately equals to the voltage on resistor 20.
This self-biased amplifier is not suitable for cellular communications because, under high power and high frequency conditions, the impedance of inductor 25 and capacitor 30 are no longer negligible. As a result, the amplitude of the negative bias is affected by the input signal. Moreover, the source of the FET is required to directly connect to ground for high frequency operations, a topology not available in this amplifier.
An improved self-biased amplifier is described in U.S. Pat. No. 5,374,899 to Griffiths et al. As depicted in FIG. 2, this amplifier includes, in addition to capacitor 20 and resistor 30, a diode 35 connected in parallel to the resistor. With increased input ac signal, the diode becomes more conductive and consequently, the variation of the source voltage is reduced. Although this circuit appears to have improved power and gain operation over the conventional self-biased amplifier, it remains a disadvantage that the source of the FET is not directly connected to ground.
Referring to FIG. 3, another known amplifier uses a portion of the input ac signal rectified by a rectifier 36 to generate a negative bias. The rectifier consists of a diode 37, a resistor 38 and a pair of capacitors. A major drawback of this amplifier is that in order to generate the negative voltage, it consumes a portion of the input RF signal, thus diminishing the already precious RF source signal. Another problem is that the rectifier generates a relatively stable negative voltage only when the input RF signal is an analog signal. When the input RF signal is digital, the amplitude of the negative voltage varies and it depends on the density of transmit data.
U.S. Pat. No. 5,327,583 to Yamada et al. describes generating a negative bias using a clock signal of a microcomputer. As depicted in FIG. 4, the clock signal is provided from a microprocessor (not shown) to a CMOS invertor and then to a circuit 39 which generates a negative voltage.
We believe this circuit has some significant drawbacks. First, due to process variations in fabricating FETs, the zero-bias saturation current of each FET varies slightly from device to device. As a result, circuit 39 appears to require individually tuning in order to provide a precise negative bias, which requires additional manufacturing steps. Second, this amplifier is not power efficient because one half of the clock signal passes through diode 45 to ground. Further, although circuit 39 includes two capacitors to smooth ripples in the negative bias, we believe that certain amount of ripples still exists which interferes with input ac signal and causes the noise level to increase. It is preferred that the ripples be reduced.
It is therefore an object of the present invention to provide an amplifier having at least one depletion-mode FET and operating with a single polarity power supply;
it is a further object to provide a negative voltage generator within the power amplifier to generate a negative voltage for biasing the FET;
it is still another object to regulate the negative voltage generated by the negative voltage generator to a desired level before providing it to the depletion-mode FET;
it is another object to reduce ripples in the negative voltage generated by the negative voltage generator;
it is still a further object to regulate the negative voltage generated by the negative voltage generator and to make it insensitive to small variation in the negative voltage; and
it is a further object to make the negative voltage regulator power efficient and particularly when the amplifier is in a stand-by mode.